The present invention relates generally to the automatic positioning and repositioning of satellite receiver antennas and more particularly relates to means for accurately referencing the revolving motion of electric motors used to position apparatus such as receiver antennas.
Television signals which are broadcast via satellite are received by TVRO receivers in conjunction with ground station antennas which are accurately focused onto the particular satellite of interest to the receiver. The antenna has to be repositioned whenever signals from a different satellite are to be received. A high degree of accuracy is required in the positioning and repositioning of such earth station antennas because mispositioning of the order of fractions of an inch may mean the difference between good reception and total loss of the received signals.
Almost all positioning devices use low voltage d-c motors to move the satellite antennas through a prescribed range of motion. These motors, which are typically in the fractional horsepower range of 0.05 to 0.15 hp and a d-c voltage range of 12 to 36 volts, are used to drive either linear screw actuators or wormdrive gear boxes linked to the mounting mechanisms for the earth station antennas.
Conventional methods for the measurement of antenna position generally use position sensors or transducers, such as "Hall effect" or "Reed" switches actually located on the mounting mechanism of the antenna. Such sensors, along with the necessary wiring for them, constitute a significant cost addition factor for the overall system, which can be critical, especially in the highly competitive market of TVRO receivers and related earth station antenna accessories.
Further, these methods are subject to limits in their positional accuracy, and the sensors and related circuits are generally limited to use with a particular motor and may require readjustments or even replacement prior to use with different positioning motors.